Look around the room. Door frames, tabletops, books, window panes — they are all built from straight lines and corners. In Year 4 we learn to describe them: which lines are parallel, which meet at a right angle, and what name a shape gets depending on how its sides and angles fit together.
Three jobs for this year
Recognise parallel and perpendicular lines — lines that never meet, lines that meet at a right angle.
Spot the right angle — and tell it apart from an acute or obtuse angle.
Classify polygons — give a triangle or quadrilateral its proper name (right triangle, square, parallelogram, …).
Words to know
Word
Meaning
line
a straight path that goes on forever in both directions
line segment
a piece of a line with two end-points
ray
a "half-line" starting at one point and going on forever
angle
the opening between two lines meeting at a point
polygon
a closed shape made of straight sides
A line drawn on a sheet of paper is really a segment (the paper ends!), but in school we usually just say "line" to keep things short.
Right angle — the most important angle
A right angle is a quarter turn. It is what you see in:
the corner of a book
the corner where two walls meet
the cross of a window frame
We mark a right angle with a small square inside the corner, like this: ⌐.
Angles smaller than a right angle are acute; angles bigger than a right angle but smaller than half a turn are obtuse.
A first taste of classification
Two shapes with the same number of sides can still get different names depending on which sides are equal and what kind of angles they have.
Triangle
What makes it special
equilateral
all three sides equal
isosceles
exactly two sides equal
scalene
all three sides different
right triangle
one right angle
acute triangle
all angles acute
obtuse triangle
one obtuse angle
A triangle can have two names at once: an isosceles right triangle has two equal sides and one right angle. We'll meet quadrilaterals with even more names on their own page.
